1. Field of the Invention
This invention relates to a magneto-resistive effect magnetic head employing a spin valve film.
2. Description of the Related Art
In keeping up with the tendency towards a smaller size and a larger recording capacity of a hard disc device, an increasing demand is raised for a small-sized hard disc device of, for example, 2.5 inch size, especially for a usage in which application to a portable personal computer is a possibility.
With such small-sized hard disc, the medium speed is lowered depending on the disc diameter, so that, in a conventional induction type magnetic head in which the playback output depends on the medium speed, the playback output is lowered to frustrate the endeavor to raise the recording capacity.
On the other hand, with a magneto-resistive effect type magnetic head (MR head) exhibiting the magneto-resistive effect, that is a magnetic head in which resistivity is changed with the magnetic field, its playback output is not dependent on the medium speed, such that a high playback output can be realized even at a low medium speed. For this reason, attention is being directed to the MR head as a magnetic head capable of realizing a high recording capacity with a small-sized disc.
The MR head is such a magnetic head for reproduction exploiting the magneto-resistive effect in which the value of the electrical resistance is varied with the direction of magnetization, and the direction of the current flowing in the head, as may be evidenced in transition metals. Specifically, should the stray magnetic flux from a magnetic recording medium be received by the MR film, the direction of magnetization of the MR film is reversed by this magnetic flux, such that the direction of magnetization is at an angle to the direction of the current flowing in the MR film which is related with the quantity of the magnetic flux. This varies the electrical resistance of the MR film and a voltage change corresponding to this variation in electrical resistance appears across the ends of the MR film through which flows the sense current.
Therefore, the magnetic recording signals can be read out with this voltage change as a voltage signal. At this time, a bias voltage is applied so that an operating point of the MR film will be a point of superior linearity in resistance change with respect to the external magnetic field and the maximum resistance change of the MR film, that is an optimum bias point.
The MR head is prepared by forming the above MR film, an electrode film or an insulating film by the thin film technique and by etching these films by a photolithographic technique. Also, with this MR head, a shield structure comprised of lower and upper magnetic poles arranged on upper and lower sides of the MR film s used in order to set a gap length during reproduction to prevent intrusion of unneeded magnetic flux to the MR film.
Specifically, with the so-called longitudinal MR head in which the sense current flows in the direction perpendicular to the track axis direction, a first insulating film, a soft magnetic film, as a lower magnetic pole, and a second insulating film, formed of Al.sub.2 O.sub.3 or SiO.sub.2, are layered sequentially. On the second insulating film is arranged a substantially rectangular MR film so that its longitudinal direction is substantially perpendicular to the magnetic recording medium sliding surface and so that its end face is exposed to the magnetic recording medium sliding surface. On both ends in the longitudinal direction of the MR film are arranged a forward end electrode an a rear end electrode for furnishing the sense current to the MR film. On the MR film is arranged an insulating film formed of, for example, Al.sub.2 O.sub.3 or SiO.sub.2. This insulating film is sandwiched between the forward end electrode and the rear end electrode. On the insulating film is arranged a bias conductor facing the MR film for applying a bias magnetic field thereto. On the bias conductor is arranged an insulating film on which a soft magnetic film is layered as an upper magnetic pole to complete the MR head.
The MR film is a film of a magnetic material displaying the magneto-resistive effect.
On the other hand, a giant magneto-resistive effect film, having a multi-layered structure combined from plural materials for exhibiting the giant magneto-resistive effect, is attracting attention. Among the giant magneto-resistive effect films, there is a spin valve film having a simpler structure and in which the electrical resistance is changed with a weaker magnetic field.
Basically, the spin valve film has a four-layered structure of a first ferromagnetic layer, a non-magnetic layer, a second ferromagnetic layer and an anti-ferromagnetic layer, arranged in this order. By separating the first ferromagnetic layer and the second ferromagnetic layer by the non-magnetic layer and by providing the anti-ferromagnetic layer on the second ferromagnetic layer, the second ferromagnetic layer contacted with the anti-ferromagnetic layer is magnetized in a pre-set direction. This second ferromagnetic layer is referred to hereinafter as a pin layer. On the other hand, the first ferromagnetic layer separated by the non-magnetic layer does not assume a fixed direction of magnetization. This ferromagnetic layer is referred to hereinafter as a free layer. That is, the pin layer and the free layer are higher and lower in coercivity, respectively.
If a magnetic field is applied across above-described the spin valve film, the free layer is magnetized to set the direction of magnetization. The resistance of the spin bulb film is maximum and minimum if the direction of magnetization of the free layer is 180.degree. opposite to that of the pin layer and if the direction of magnetization of the pin layer is the same as that of the pin layer, respectively.
However, with the MR head employing a spin valve film, magnetic domain control of the free layer is crucial. The free layer devoid of a preset direction of magnetization is of a magnetic domain structure in which the magnetic pole is not exposed to outside, as shown in FIG. 1, in order to avoid increase in magnetostatic energy in the absence of the effect of an external magnetic field. If, in a state in which there are a large number of magnetic domains of different directions of magnetization in the free layer of the spin valve film, an external magnetic field is applied across the free layer, the directions of magnetization are moved in unison to render uniform the directions of magnetization in the free layer. Since the magnetic wall is destroyed during transition from the multiple magnetic domains to the unitary magnetic domain, there is produced the noise termed the Barkhausen noise which is applied to an output waveform.